1. Field of the Invention
The invention is related to the field of production logging of oil and gas wells. More specifically, the invention is related to methods and apparatus for measuring the volumetric flow rates of water in highly inclined or horizontal wellbores, where in portions of such wellbores some of the water may be flowing in a direction away from the wellhead.
2. Description of the Related Art
Highly inclined and horizontal wellbores are drilled for the purpose of more efficiently extracting petroleum from reservoirs in earth formations than is generally possible using vertical wellbores. The increase in efficiency is related to the length of the wellbore which penetrates the petroleum-bearing portion of the earth formation. Since many petroleum reservoirs are segregated by gravity and have large lateral extent with respect to their vertical aspect, highly inclined or horizontal wellbores enable having a very long wellbore length through the petroleum-bearing portion of the reservoir.
The typical highly inclined or horizontal wellbore does not penetrate straight through the reservoir, however. When a highly inclined or horizontal wellbore is drilled, the drilling operator typically will have to make adjustments to the drilling direction to maintain the wellbore trajectory within the desired part of the particular reservoir. As a result of these trajectory adjustments, the typical highly inclined or horizontal wellbore will undulate about the intended trajectory and therefore can have sections which are inclined more than 90.degree. deviation from vertical. This means that while the measured depth of the wellbore (this usually being defined as the length of the wellbore from its termination at the earth's surface at a "wellhead") is increasing, the true vertical depth (defined as the absolute vertical distance to the earth's surface from any point in the wellbore) is actually decreasing over such sections of the wellbore. As the wellbore trajectory is returned to the desired position within the reservoir, the inclination may be "dropped" by the drilling operator to continue drilling the wellbore at a particular true vertical depth, creating an undulation. The undulated sections of the wellbore having segments at above 90.degree. inclination can form gravity "traps" for more dense fluids such as water, which may be produced from the reservoir when the wellbore is completed.
Various production logging methods are known in the art for identifying zones in a reservoir (or the reservoirs in a wellbore which penetrates multiple reservoirs) which cause water to be introduced into the wellbore. The production logging methods known in the art enable the wellbore operator to determine the volumetric flow rates of water into the wellbore from any interval traversed by the production logging instrumentation. Most production logging methods known in the art, which include various velocity measuring devices such as the "spinner" flowmeter, do not easily resolve whether some of the water in a horizontal wellbore is stagnant or flowing "backwards", that is, in a direction away from the wellhead. Such a flow direction is possible particularly near the previously described "traps", in the segments of the wellbore having inclination above 90.degree.. The limitations on prior art production logging methods are primarily because the velocity measurements made by devices such as the spinner flowmeter are localized to the position of the instrument itself within the cross-section of the wellbore. Near traps, some of the water may be flowing away from the wellhead due to gravity, while other portions of the water may be flowing towards the wellhead. The true volumetric flow rate of water towards the wellhead may not be correctly measured by spinner flowmeters or other "localized" velocity measuring devices.
One method for determining the volumetric flow rate of water irrespective of internal turbulences, or the presence of other fluids in the flow stream such as oil and gas, is known as "oxygen activation" logging. Oxygen activation logging is described, for example, in U.S. Pat. No. 5,461,909 issued to Arnold or in U.S. Pat. No. 5,404,752 issued to Chace et al. Generally speaking, the method described in these patents includes bombarding the fluid in the wellbore with bursts of high energy neutrons, and detecting gamma rays which are characteristic of oxygen which has been "activated" by the high energy neutrons. The rates at which the gamma rays are detected at one or more detectors spaced apart from the neutron source is related to the velocity of the water moving past the logging instrument.
A method for determining the flow rate of water where there is more than one "phase" ("phase" referring to a fluid component such as oil or gas) in the fluid moving through the wellbore is described in U.S. Pat. No. 5,306,911 issued to Hunt. The method in the Hunt '911 patent includes measuring the oxygen-activation gamma rays at a fixed position within the wellbore for a period of time and determining the water flow rate by characterizing the time-based measurements of gamma rays with respect to water flow rate. A significant limitation of the method described in the Hunt '911 patent is that it requires keeping the logging instrument stationary within the wellbore to make the time-based gamma ray measurements. In a horizontal or highly inclined wellbore where several thousand feet of wellbore may need to be evaluated to determine the zones causing the water production, the method in the Hunt '911 patent would be impracticable.
What is needed is a method for measuring flow rates of water in highly inclined or horizontal wellbores which enables substantially continuous movement of the logging instrument through the wellbore.